[Illustration: Fig. 16. _Cynosurus cristatus._ Transverse section of the leaf-blade (× 20). Stebler.] The ridges are almost always evident--_Catabrosa_, _Poa_, and _Avena_ furnishing the chief exceptions--and are nearly invariably on the upper surface: they are below in _Melica_, however; and their relative numbers, heights and breadths, section--acute, rounded, or flattened--furnish valuable characters; as also does the coexistence or absence of hairs, asperities, &c. [Illustration: Fig. 17. Transverse section of the leaf of _Festuca elatior_, var. _pratensis_ (× 12).] [Illustration: Fig. 18. Ditto of the leaf of _F. ovina_ (× 15).] [Illustration: Fig. 19. Ditto of the leaf of _F. ovina_, var. _rubra_ (× 35).] [Illustration: Fig. 20. _Festuca ovina_, var. _rubra_. Transverse section of the blade of an upper leaf (× 35). Stebler.] A very interesting anatomical adaptation is met with in the leaves of many grasses which grow in dry situations (xerophytes) such as on sandy sea-shores, exposed mountains and so forth. When the air is moist, in wet weather or in the dews, and the sun’s rays not too powerful, the leaf is spread out with its upper surface flat or nearly so, but when the scorching sun and dry air or winds prevail, the leaves fold or roll up, with the upper sides apposed or overlapping inside the hollow cylinder thus made. In such leaves some of the upper epidermal cells, either next the mid-rib (_Sesleria_ &c.) or between the other ribs (_Festuca_ &c.) are large and very thin-walled, full of sap when distended, and so placed that as they lose water by evaporation they contract, and so draw together the two halves of the lamina (_Sesleria_) or each ribbed segment (_Festuca_), thus causing the infolding or inrolling (see Chapter IV). Not only from the structure and actions of these motor-cells, but also from the fact that the stomata are on the upper surfaces and thus protected, and that the lower surfaces which alone are exposed to the drought are defended by hard and impenetrable tissues, we must look upon these as adaptations to the xerophytic conditions. Leaves prominently ridged. _Elymus._ _Psamma._ _Aira cæspitosa._ _Lolium._ _Cynosurus_ (Fig. 16). _Agrostis._ _Alopecurus._ _Glyceria fluitans._ _Kœleria._ _Festuca elatior._ _Festuca Myurus_ (var. _sciuroides_). _Melica_ has ridges on the _lower_ surface. Ridges are less prominent in _Phleum pratense_, _Briza_, _Agropyrum_, _Triodia_, _Arrhenatherum avenaceum_. Leaves practically devoid of ridges. _Poa_--all common species. _Glyceria aquatica._ _Catabrosa aquatica._ _Avena pratensis._ In some grasses the tissue over the mid-rib is considerably raised and strengthened on the dorsal side of the blade as a “keel.” Keel more or less prominent.